Does Fasting Burn Muscle? [The Full Story, Backed by Science]

Does fasting burn muscle

Have you ever been told that if you fast, you’ll lose all your muscles?

That’s a common fear. But don’t worry, it’s mostly nonsense. 😉

Case in point:  Wolverine.  

In case you didn’t know, Hugh Jackman used intermittent fasting for muscle gain when he was getting ripped for the X-Men movies.  Obviously, fasting didn’t consume his muscles–quite the opposite.  

Fasting and muscle loss is just one of those myths people like to repeat.  We’ve all heard it, so don’t feel bad if you bought into it at some point.  

The bottom line?

Fasting burns a lot more fat than anything else.

Today I’ll share with you the logic and science of why fasting doesn’t “burn” muscle. 

I’ll also share what you can do to effectively maintain (& even build) your muscle strength when you fast.

Let’s get started.  

First of All – Your Body Isn’t Stupid

To start with, your body isn’t that illogical. It doesn’t tear down your muscles when it has tons of other energy available, as body fat. 

Dr Fung (author of multiple best-selling books about fasting and nutrition) explains:

“If you actually believe that, you’d have to believe that mother nature designed our bodies to store food energy as fat and sugar.  But when we actually need it, to burn protein.  That sounds pretty dumb.” 

–Dr. Jason Fung

Our ancestors did a LOT of fasting.  So if it burned up their muscles they would have turned into blubbery blobs that couldn’t move.  Obviously if that had happened, we wouldn’t be alive today.  

Or to borrow an analogy from Doctor Fung’s video, if you had a sofa and a bunch of firewood, when the time came to start a fire, would you chop up your sofa before using the firewood?

That’s basically the equivalent of saying the body will burn muscle (instead of fat) when you’re fasting.  

Glycogen Sustains Your Blood Sugar During Intermittent Fasting

Most of the time, you have chains of sugar (glucose) stored in your liver.  The chains are called “glycogen”.   

You fill your glycogen storage tank whenever you eat carbs.  Once filled, it can maintain your blood sugar for quite a while — roughly 12-24 hours.  

Why does this matter?

It means if you’re just doing daily intermittent fasting (a.k.a time-restricted eating), you’ll probably never run out of sugar, in your liver.

Glycogen Sustains Blood sugar during intermittent fasting

In other words, your body won’t even need to look for other potential sources of blood sugar (like protein), because you still have plenty of sugar stored from your previous meal.

(If you were doing a strict low-carb diet, this would be a slightly different scenario.  But for most people most of the time, the timeline in the image above applies.)

The bottom line?

Daily intermittent fasting is not a threat to your muscles.

Gluconeogenesis Uses Protein and Fat to Make New Sugar

Your liver does this fancy thing where it makes blood sugar out of other substances, including protein.

That’s called gluconeogenesis.

If you break that word down, perhaps you can tell it means “making new sugar”.

There’s a particular phase of fasting when you become more reliant on gluconeogenesis to sustain your blood sugar — the “transition” phase

You rely on gluconeogensis during the transition phase of fasting

Protein Comes From Various Sources

Most people assume your body will break muscle protein to make more blood sugar during that “transition”.      

Here’s the thing:

Muscle isn’t the only type of protein in your body.  There’s a bunch of connective tissue and other structures that have lots of protein as well.  

So your liver doesn’t just use muscle protein for gluconeogenesis.

Autophagy Finds Protein to Recycle, Throughout Your Body

Your body also “scavenges” inside cells to find worn out or defective proteins.  That’s called autophagy. Your liver then uses some of that protein to make sugar.  

Protein from autophagy can help prevent muscle loss

Autophagy starts ramping up around the same time you start relying on gluconeogenesis to maintain your blood sugar. 

Coincidence?  I think not.  

Related Post: The 5 Stages of Fasting [Benefits + How to Make Easier]

Making Blood Sugar Out of Fat?!

Little known fact:  

You can also make more blood sugar out of fat!  Not all of the fat, but particularly this one molecule called “glycerol”, which is part of a “triglyceride” (glycerol + 3 fatty acids, hence the name).  

As you burn more and more fat, you also have more and more glycerol to convert into blood sugar.

Bottom line?

When your body needs protein, there are lots of other places to find it besides muscle.

You may use a little bit of protein from muscle during the “transition phase”, but not that much.  And you can easily rebuild it later (more on that below).

And don’t forget, since you’re burning lots of fat, you’ll have quite a lot of glycerol to convert into glucose as well.  

Growth Hormone Preserves (and Rebuilds) Muscle When You Fast

While you fast, your body makes more growth hormone.

As you can probably tell from the name, growth hormone stimulates growth. 🙂  Luckily, that includes muscle growth.

Growth hormone helps preserve and rebuild muscles when you fast

Growth hormone helps preserve your muscles while you fast.  It also helps rebuild any muscle you may have lost after you fast — and then some.

End result?

When done properly (including adequate protein intake before and after), fasting results in a net gain of muscle, rather than muscle loss.

Ketones Directly (and Indirectly) Preserve Muscle Mass

One of the many awesome things about ketones is that they help preserve muscle mass. 

Your liver makes ketones from fat, and ketones send a signal to your body not to break down muscle.  So you preferentially use other proteins instead–like connective tissue, and all that protein from autophagy.

And don’t forget-

The more ketones you have, the less blood sugar you need anyway.  

For example, after a few days of continuous fasting, my ketones go sky high.  At that point, my blood sugar typically drops to what would be considered a “dangerously” low level (like 40 or 50 mg/dL), if I weren’t fasting.  

That’s because my brain is then running on ketones, instead of sugar.  So I don’t need much sugar anymore.

Here’s a related hack:

If you eat a ketogenic diet for a week or so before a prolonged fast, you can get your ketones up in advance.

a ketogenic diet helps preserve muscle during extended fasting

That not only makes it easier to fast, it also helps preserve your muscle mass, by replacing your blood sugar with ketones from the very start.  

Interestingly, the more your body adapts to using ketones, the better ketones get at preserving muscle mass.  Yet another reason to take things slow when you’re starting out with fasting or a ketogenic lifestyle.  

Exercise Promotes Muscle Growth…Even While You’re Fasting!

It turns out, you don’t even need to eat anything to build muscle.  

Sure, you need to eat some protein eventually.  

But exercise promotes muscle growth even when you don’t eat

“The effect of exercising alone actually stimulated muscle protein synthesis, even in the fasted state, for up to 2 days.”  

–Stuart Phillips, professor of Kinesiology (Stem-talk podcast episode 82)

Where does it get the protein to do that?  Autophagy, for one.  

Here’s the bottom line:

As long as you keep using your muscles while you fast (through moderate daily exercise), they won’t get significantly smaller.  So don’t be a couch potato. 🙂

You Can Build More Muscle When You Break Your Fast

When you end your fast, that’s an excellent opportunity to stimulate muscle growth. 

Not only will you be introducing protein back into the body, your growth hormone will still be elevated from fasting.

So do some resistance training, feast on protein, and build those muscles!

Related Questions

To round things out, here are a few related questions.

Can You Build Muscle While Fasting?

That depends on what you mean.

You’re not going to build a lot of muscle during your actual fasting window. But for all the reasons specified above, you can do a pretty good job of at least preserving muscle while you fast.

Most of the muscle gets built as soon as you stop fasting.  In other words, during the refeeding phase.

If you’re just doing short-term daily fasting, you refeed every single day.  So there’s more than enough opportunity for muscle growth.  

If you do an extended fast, you can essentially maintain your muscle while you’re fasting, and then build additional muscle as soon as you stop fasting — by exercising and eating protein.

How Long Can You Fast Without Losing Muscle?

That depends on how much body fat you have.

If you have a lot, you can fast a long time before you would lose any significant amount of muscle.

In other words, you would basically have to run out of body fat before you’d start “burning” muscle.  

I don’t know about you, but I have a LONG way to go before that would happen!

And don’t forget-

As I mentioned in an article about why fasting works well for weight loss, you’ll actually burn more muscle by doing a calorie restriction diet than you will by fasting.

How Can You Fast Without Losing Muscle?

If you’ve been reading up till now, you probably already know the answer.  

Regardless, here’s a summary of how to do a “muscle sparing fast”:

During short-term daily fasting (a.k.a. intermittent fasting), glycogen stored in your liver can maintain your blood sugar. So your muscles are safe.  

During a prolonged water fast, there are 3 main things you can do to avoid significant muscle loss:

First, Get Your Ketones Up in Advance.  

Ketones will naturally become elevated after a couple days of fasting, but ideally you want them high as soon as possible.  

That’s because ketones directly inhibit muscle breakdown. They also take the place of blood sugar so you don’t need to make as much of it anymore.

Second, Exercise During & After Your Fast

Exercise preserves (and even slightly builds) muscle while you’re fasting.  And it can help build a lot more of it when you finish fasting.

Third, Feast on Protein

Whether you’re doing a short fast or a long fast, it’s always important to “feast” afterwards.  

That means eat plenty of food, especially protein.

In other words, don’t try to “diet” in between fasts.  It’s not only unnecessary, it’s typically counterproductive.   

(Related Note:  After several days of prolonged fasting, don’t break your fast with a big meal right away.  Eat something small, wait a few hours, and go from there.  Otherwise you’ll get an upset stomach, and potentially other problems.)

Summary & Final Thoughts

Does fasting burn muscle?

That’s a common myth that you’ve probably heard, but it’s also a load of horse manure. 😉  

Humans have been fasting for thousands of years, and along the way our bodies learned how to handle it.  In fact, they thrive on it.  

Our ancestors did a LOT of fasting.  And their muscles didn’t turn to mush, or we would’ve never been born.  

During short-term fasting (up to about 24 hours), glycogen in your liver can easily maintain your blood sugar.  Easy peasy.  

During long-term fasting, you may use a tiny bit of muscle protein as you transition into fat-burning mode.  But you’ll also use protein from other sources, along with glycerol from fat.  

Multiple other mechanisms help preserve and rebuild your muscles during and after a prolonged fast.  These include ketones (direct and indirect effects), growth hormone, and physical movement.    

The end result?

Fasting + feasting + exercise = a net increase in muscle mass.

So quit your worrying, and get started. 🙂

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Picture of Ben Tanner, PA-C

Ben Tanner, PA-C

Ben has been practicing as a physician assistant (PA, or PA-C, similar to a doctor) in emergency medicine, urgent care, and family practice since 2014. Since 2016, he has developed an avid interest in various forms of fasting, using it to improve his own health while helping friends, family, and patients do the same.